This image was taken by ROSAT in June of 1990 when the Moon was about half-full. Chandra's observations of the Moon solved a decade-long mystery about X-rays detected by ROSAT that were thought to be coming from the dark portion of the Moon. It turns out that these X-rays only appear to come from the Moon and can be explained by radiation from Earth's geocorona. (Credit: ROSAT/J. Schmitt et al.)